Since there exists an apparent correlation between bacterial dental plaque and gingival inflammation, fundamental research is essential to define the bacterial composition of gingival and subgingival plaques and their biological implications in periodontal disease. To expediently deal with these research problems, a concerted team effort by microbiologists, immunologists, experimental pathologists and clinicians is under way. Bacterial succession during subgingival plaque formation is being characterized in man in combination with clinical and ultrastructural analysis of concomitant tissue responses. The effects of autologous, suspect pathogenic microorganisms, other plaque constituents and host specific factors upon the inflammatory reactions are also investigated. Specific in vitro and in vivo methods are being employed to study various immunological parameters where healthy and periodontally diseased individuals are challenged by their own plaque. Direct (vital) microscopy combined with subsequent ultrastructural analysis are also carried out on the free gingiva of dogs and of man as well as in the rabbit ear chamber to establish in situ verifications.